
Field of the Invention
The present invention relates generally to rotation angle sensors for sensing a rotation angle of a rotatable member, and more particularly to a rotation angle sensor for determining an opening degree of a throttle valve or accelerator employed in an automobile engine.
Rotation angle sensors are generally used for determining an opening degree of a throttle valve or accelerator of automobile engines. The conventional method of determining the opening degree of the throttle valve by using the rotation angle sensors will be explained hereinafter.
Automobile engines with electronically controlled fuel injection system generally include a throttle valve installed in an air intake duct. A rotation angle sensor is provided for detecting an opening degree of the throttle valve. Electrical signal generated from the rotation angle sensor is transmitted to a control unit and processed as output signal indicative of an intake air flow. The control unit computes an amount of fuel injection depending on the intake air flow. As the conventionally proposed rotation angle sensors for automobile engines, there are known those of a non-contact type using a magnet and a yoke around the magnet.
U.S. Pat. No. 5,811,968 discloses a rotation angle sensor including a magnet attached to a rotatable shaft of a throttle valve. An opening degree of the throttle valve varies depending upon the rotation of the shaft. At least three yokes surround the magnet. A plurality of Hall effect devices are disposed between the yokes. The magnet and the yokes cooperate with each other to constitute a plurality of magnetic circuits. Each of the Hall effect devices is incorporated within each magnetic circuit. The plurality of Hall effect devices detect magnetic flux densities in the plurality of magnetic circuits and generate output signals indicative of the magnetic flux densities detected. A rotation angle of the shaft of the throttle valve, i.e., as an opening degree of the throttle valve is determined by using the output signals generated from the Hall effect devices.
Multiple yokes such as the three or more yokes used in the above-described conventional rotation angle sensor, become complicated in configuration because these yokes have multiple bent or cutout portions. Magnetic flux generated by the magnet tends to leak out through the bent or cutout portions of the yokes when the magnetic flux passes into the yokes. Further, in a case where the multiple yokes are arranged along substantially the entire circumference of the magnet with a less space between the respective adjacent two of the multiple yokes, a circumferential leakage flux occurs between the adjacent yokes. This adversely affects the detection results of magnetic flux densities by the Hall effect devices and, therefore, results in inaccurate output signal therefrom. In addition, the arrangement of the multiple yokes within a casing will lead to increase in size of the rotation angle sensor as a whole, number of the components and then the production cost.